Performance of an integrated circuit, such as an image sensor, can depend on the temperature. For example, the “dark current” inside an image sensor—i.e., the unwanted current produced by the sensor even during periods when it is not actively exposed to light—is highly temperature-dependent. The dark current will increase with increasing temperature, and higher dark current levels degrade the performance of the image sensor. In particular, as the dark current rises, the dynamic range of the image sensor diminishes and the dark reference level wanders or becomes uncertain, since current flows regardless of the ambient darkness. As a result, various defects may appear in captured images, and if the temperature becomes too high the sensor may sustain permanent damage. Accordingly, the ability to monitor the temperature of an image sensor may be crucial not only to detect and compensate for temperature-induced anomalies, but to protect the sensor from damage.
One conventional technique for measuring the temperature of an image sensor is to mount a thermal couple on the package of the image sensor, either at the front side or at the back side of the package, depending on how the sensor is arranged on the circuit board. The thermal couple can occupy a significant area, however, increasing the size and cost of the sensor and complicating its integration into an image-capture device, such as a camera. Also, over time, the epoxy used to affix the thermal couple to the package can age and loosen, in which case the temperature typically cannot be measured until the loose epoxy is repaired.
Furthermore, different devices may exhibit very different temperature sensitivities. Accordingly, the actions taken in response to a particular temperature reading will be device-specific. Knowing the device temperature, in other words, is insufficient to determine the optimal action to be taken without knowledge of the device and its response to, and tolerance of, temperature variations.